Power conditioning units

ABSTRACT

We describe a power conditioning unit with maximum power point tracking (MPPT) for a dc power source, in particular a photovoltaic panel. A power injection control block has a sense input coupled to an energy storage capacitor on a dc link and controls a dc-to-ac converter to control the injected mains power. The power injection control block tracks the maximum power point by measuring a signal on the dc link which depends on the power drawn from the dc power source, and thus there is no need to measure the dc voltage and current from the dc source. In embodiments the signal is a ripple voltage level and the power injection control block controls an amplitude of an ac current output such that an amount of power transferred to the grid mains is dependent on an amplitude of a sinusoidal voltage component on the energy storage capacitor.

CLAIM OF BENEFIT TO PRIOR APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 13/843,772, filed Mar. 15, 2013, now published asU.S. Publication 2013/0279210. U.S. patent application Ser. No.13/843,772 is a continuation application of U.S. patent application Ser.No. 12/789,154, filed May 27, 2010, now issued as U.S. Pat. No.8,405,367. U.S. patent application Ser. No. 12/789,154 is acontinuation-in-part application of U.S. patent application Ser. No.12/160,743, filed May 2010, now issued as U.S. Pat. No. 8,089,785. U.S.patent application Ser. No. 12/160,743 is a national stage applicationof International Patent Application PCT/GB2007/050014, filed Jan. 12,2007, now published as WO 2007/080429. International Patent ApplicationPCT/GB2007/050014 and U.S. patent application Ser. No. 12/160,743 claimthe benefit of United Kingdom Patent Application GB 0600658.9, filedJan. 13, 2006. U.S. patent application Ser. No. 12/789,154 claims thebenefit of United Kingdom Patent Application GB 1004621.7, filed Mar.19, 2010. U.S. patent application Ser. No. 13/843,772, now published asU.S. Publication 2013/0279210 and U.S. Pat. No. 8,405,367 areincorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to power conditioning units for delivering powerfrom a dc power source to an ac output, either for connecting directlyto the mains (grid) utility supply or for powering mains (grid) devicesdirectly, independent from the mains utility supply. More particularlythe invention relates to methods and apparatus for Maximum Power PointTracking (MPPT) for such power conditioning units, in particular forphotovoltaic panels.

BACKGROUND TO THE INVENTION

We have previously described (see above) improved techniques for energycontrol in power conditioning units (inverters). In particular thesetechniques enable the construction of a dc-to-mains power converterwhich does not need to use electrolytic capacitors, which are prone tofailure, especially at high temperatures as encountered, for example,behind a solar PV (photovoltaic) panel.

Background Prior Art can be Found in:

EP 0780750A; JP 2000020150A; US 2005/0068012; JP 05003678A; GB2415841A;EP0947905A; WO2006/011071; EP1,235,339A; WO2004/006342; DE 100 64 039 A;US2005/030772; WO96/07130; U.S. Pat. No. 6,657,419; US2004/117676;US2006/232220; WO2004/001942; GB2419968A; U.S. Pat. No. 7,319,313; U.S.Pat. No. 7,450,401; U.S. Pat. No. 7,414,870; U.S. Pat. No. 7,064,967;“Cost-Effective Hundred-Year Life for Single-Phase Inverters andRectifiers in Solar and LED Lighting Applications Based on MinimumCapacitance Requirements and a Ripple Power Port”, P. T. Kerin and R. S.Balog—technical paper; US2009/0097283; “Long-Lifetime Power Inverter forPhotovoltaic AC Modules”, C. Rodriguez and G. A. J. Amaratunga, IEEETrans IE, 55(7), 2008, p 2593; US2008/097655.

We will now describe some improved techniques for maximum power pointtracking, which are particularly suitable for use with powerconditioning units of the types we have previously described.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is therefore provideda power conditioning unit with maximum power point tracking (MPPT), fordelivering power from a dc power source to an ac mains power supplyoutput, the power conditioning unit comprising: an input for receivingpower from said dc power source; an output for delivering ac power tosaid ac mains power supply; an energy storage capacitor for storingenergy from said dc power source for delivering to said ac mains powersupply output; a dc-to-ac converter coupled to said output forconverting energy stored in said energy storage capacitor to ac powerfor said ac mains power supply output; a power injection control blockhaving a sense input coupled to said energy storage capacitor and havingan output coupled to said dc-to-ac converter, to control said dc-to-acconverter to control power injected into said ac mains power supply; andwherein said power injection control block is configured to track amaximum power point of said dc power source without measuring a dcvoltage or dc current provided from said dc power source.

In some preferred embodiments a voltage on the energy storage capacitorhas a sinusoidal voltage component (at twice the frequency of the acmains), and the power injection control block is configured to controlan amplitude of an ac current provided to the ac mains power supplyoutput such that an amount of power transferred to the output isdependent on an amplitude of the sinusoidal voltage component on theenergy storage capacitor. In embodiments the average energy transferredis linearly dependent on, more particularly proportional to, a squaredvalue of the sinusoidal voltage component. In embodiments the sinusoidalvoltage component is superimposed on a dc link voltage (input to thedc-to-ac converter), and this link voltage is relatively high, forexample less than 200, 300, 400 or 500 volts. In such an embodiment theaverage power transferred is proportional to the difference between thepeak (maximum) capacitor voltage squared and the trough (minimum)capacitor voltage squared (although alternatively a power conditioningunit may be arranged such that there is, on average, zero dc voltage onthe energy storage capacitor). In embodiments the instantaneous powertransferred to the ac mains power supply output is dependent on orproportional to the instantaneous value of voltage on the energy storagecapacitor.

In a related aspect of the invention, therefore, there is provided apower conditioning unit with maximum power point tracking (MPPT), fordelivering power from a dc power source to an ac mains power supplyoutput, the power conditioning unit comprising: an input for receivingpower from said dc power source; an output for delivering ac power tosaid ac mains power supply; an energy storage capacitor for storingenergy from said dc power source for delivering to said ac mains powersupply output; a dc-to-ac converter coupled to said output forconverting energy stored in said energy storage capacitor to ac powerfor said ac mains power supply output; a power injection control blockhaving a sense input coupled to said energy storage capacitor and havingan output coupled to said dc-to-ac converter, to control said dc-to-acconverter to control power injected into said ac mains power supply; andwherein, in operation, a voltage on said energy storage capacitor has asinusoidal voltage component at twice a frequency of said ac mains;wherein said power injection control block is configured for controllingan amplitude of an ac current provided to said ac mains power supplyoutput such that an amount of power transferred to said ac mains powersupply output is dependent on an amplitude of said sinusoidal voltagecomponent on said energy storage capacitor, and wherein said powerinjection control block is configured to track a maximum power point ofsaid dc power source by controlling said dc-to-ac converter.

In embodiments of the above described power conditioning units an energyflow from the dc power source to the energy storage capacitor issubstantially proportional to an amount of energy change in the energystorage capacitor (this is explained further below). Further, an amountof energy drawn from the energy storage capacitor and provided to the acmains output is controlled by the power injection control block suchthat the amount of ac power delivered to the ac mains power supply isdependent on the amount of energy stored in the energy storagecapacitor. In such an arrangement the power arrangement control block isthereby able to track the maximum power point of the dc power source bycontrolling the ac power delivered to the AC mains power supply bycontrolling the dc-to-ac converter, without the need for MPP tracking onthe front end of the power conditioning unit, which typically includes adc-to-dc converter. In broad terms the power injection loop pulls power,in the first instance, from the dc power source and delivers this intothe energy storage capacitor. In the second instance the power injectionloop extracts power from the energy storage capacitor and delivers thisto the AC output. The need to deliver AC power to the output results ina sinusoidal voltage component on the energy storage capacitor, and thisis an intrinsic part of this control loop; typically this fluctuatingsinusoidal component of (a generally dc) voltage on the energy storagecapacitor has, in operation, a peak amplitude of at least 10 Volts, 20Volts, 30 Volts, 40 Volts, 50 Volts, 60 Volts or 100 Volts. The peakamplitude of this sinusoidal voltage component depends upon the currentinjected into the ac mains output.

Were MPPT to be implemented at the dc input end of the powerconditioning unit, for example by means of a control loop on a front enddc-to-dc converter, an MPPT tracking algorithm would generally impose adegree of ripple on the dc input voltage to the power conditioning unit,in order that the operating point of the dc power source can be variedto hence determine the maximum power operating point. The operatingpoint automatically adjusts according to the energy change in the energystorage capacitor. By contrast in embodiments we employ a “pull”arrangement in which power flows from the dc power source into theenergy storage capacitor in effect on demand, the demand beingcontrolled by the second, power injection control loop.

In more detail, the degree of ripple on the DC link, more particularlythe ripple amplitude, is effectively a measure of the amount of powerdrawn from the DC input, for example a solar photovoltaic panel. If theripple reduces this implies that less power is being provided from theDC input and in broad terms the power injection control block thenresponds by reducing the current injected into the grid, that is byadjusting the power injection. In embodiments the current is regulatedby adjusting the switching speed (rate) of the output DC-to-ACconverter. When the system is tracking the maximum power point, if thepower from the DC input reduces, the ripple reduces and the switchingspeed of the converter is adjusted downwards, to inject less currentinto the grid. This brings the operating point back towards the maximumpower point and balances the amount of power provided by the DC sourcewith that being injected into the grid. The control block thenperiodically increases the switching speed of the power injection blockwith the aim of increasing the amount of current flowing into the grid.This has the effect of increasing the ripple in the event that theamount of energy being provided by the DC source is greater than thatbeing harvested, and hence the control loop effectively operates so asto maximise the ripple and therefore harvested energy. In terms of atypical I-V characteristic (see FIG. 9, later) this corresponds toserving around the maximum power point, more particularly moving alongthe characteristic curve in a direction of decreasing current andincreasing voltage (as in the just mentioned example), or increasingcurrent and decreasing voltage, towards the maximum power point.

In embodiments the power injection control block generates a template ofthe AC current injected into the mains. More particularly the templatecomprises a sinusoidal or half-sinusoidal voltage in phase with the gridmains and the amplitude of this template is adjusted dependent on themeasured DC link ripple voltage, more particularly dependent on whetherthis has previously gone up or down. Thus in embodiments the amplitudeof this template signal is responsive to the ripple voltage on theenergy storage capacitor/DC link. An error signal dependent on thedifference between the measured AC current injected into the grid mainsand this template is used to control the switching rate of the powerinjection control block. In embodiments the error signal is used toincrease the switching rate if the template magnitude is greater thanthe magnitude of the current injected into the AC mains. In this way thecurrent injected is controlled with the aim of maximising the energystorage capacitor/DC link ripple.

As previously mentioned, in some preferred implementations the rippleamplitude at the energy storage capacitor/DC link is used to effectivelymeasure power provided from the DC source (photovoltaic panel). Howeverin principle other techniques may be employed to measure, at the energystorage capacitor/DC link, the power provided from the DC power source.For example absent losses the power provided by the power source may beassumed to be given by the product of voltage on and current through theDC link providing an input to the DC-to-AC converter. Nonetheless,because preferred implementations of our power conditioning unit have aripple which is proportional to input power (assuming input and outputpower are substantially the same), measuring the ripple is anadvantageous technique for obtaining the desired power information.

In a further aspect, therefore, the invention provides a method ofmaximum power point tracking (MPPT) in a power conditioning unit fordelivering power from a dc power source to an ac mains power supplyoutput, the power conditioning unit including an energy storagecapacitor for storing energy from said dc power source for delivering tosaid ac mains power supply output, the method comprising: tracking amaximum power point of said dc power source by controlling a dc-to-acconverter converting energy stored in said energy storage capacitor toac power for said ac mains power supply input, wherein said trackingcomprises: sensing, at a circuit node coupled to said energy storagecapacitor, a signal responsive to a level of power drawn from said dcpower source; and controlling said dc-to-ac converter to adjust anamplitude of an ac output to substantially maximise said sensed signal.

In embodiments the signal on the energy storage capacitors/DC link issensed and used to derive a control (template) signal having anamplitude dependent on the level of power drawn from the DC powersource, more particularly on a change in this sensed level of power.Then the switching rate of the DC-to-AC converter is controlled based ona difference between the sensed AC current and this control signal, moreparticularly increasing the switching rate of the output converter ifthe control signal (template) is greater than the sensed AC currentsignal, and vice versa.

In some preferred embodiments a dc voltage amplification stage isincluded between the dc power input and the ac mains output, and thisstage has a substantially constant amplification factor—that is it isnot varied by a control loop to perform MPPT although, in embodiments,the constant amplification factor may be selectable, for exampleaccording to the operating environment. In embodiments a voltageamplifier control block may be provided, but not to provide a variablevoltage amplification control loop but instead to act effectively as apower switch to switch on and off a path for power flow from the inputto the dc-to-dc converter. Optionally, depending upon the implementationof the voltage amplification stage, the voltage amplifier control blockmay provide a (substantially constant duty cycle) pulse width modulationcontrol signal to the dc voltage amplifier.

An arrangement of the type described above facilitates galvanicisolation between the dc input and ac mains power supply output sincethe MPPT tracking may be performed without any direct connection to thedc input for measuring voltage and/or current from the dc power source.

In an example implementation the dc-to-ac converter may comprise a buckstage converter or alternatively, for example, an “unfolding bridge” incombination with a pair of power switching devices and an outputinductor may be employed, as described in our U.S. Pat. No. 7,626,834(hereby incorporated by reference in its entirety). In embodiments thepower injection control block may be configured to sense a voltage onthe (dc link) energy storage capacitor, to scale this down, and tomultiply this by a sine wave (of appropriate phase) to create a templatesignal for comparison with a sensed signal derived from the grid mains,in order to control the output current of the dc-to-ac converter. Inembodiments no dc current sensing need be performed. In embodiments thepower conditioning unit may include an anti-islanding function, forexample as described in our co-pending U.S. application Ser. No.10/555,803 (WO2004/100348) (hereby incorporated by reference in itsentirety).

In preferred applications the above power conditioning unit is employedin conjunction with one or more photo voltaic devices (solarpanels)—that is it is a solar inverter. In some preferred embodimentsthe device is employed in combination with a single solar panel and thushas a relatively low dc voltage input, although the technique may alsobe employed with a so-called “string” of solar panels, in which case thedc input voltage may be one or several hundred volts (in such a case theamplification factor of the dc voltage amplifier may be equal to, oreven less than unity, although in general it will be greater thanunity). Potentially techniques of the type we describe may also beemployed with other types of dc power source, for example one or morefuel cells. In some preferred embodiments the energy storage capacitoris a non-electrolytic capacitor, for example a film, polyester, orpolypropylene capacitor; the capacitor may have a value of less than 50μF, 40 μF, 30 μF, 20 μF or 10 μF.

In a related aspect the invention provides a method of maximum powerpoint tracking (MPPT) in a power conditioning unit for delivering powerfrom a dc power source to an ac mains power supply output, the powerconditioning unit including an energy storage capacitor for storingenergy from said dc power source for delivering to said ac mains powersupply output, the method comprising: tracking a maximum power point ofsaid dc power source by controlling a dc-to-ac converter convertingenergy stored in said energy storage capacitor to ac power for said acmains power supply input, wherein said tracking is performed withoutmeasuring a dc voltage or dc current provided from said dc power source.

In a still further related aspect the invention provides a method ofmaximum power point tracking (MPPT) in a power conditioning unit fordelivering power from a dc power source to an ac mains power supplyoutput, the power conditioning unit including an energy storagecapacitor for storing energy from said dc power source for delivering tosaid ac mains power supply output, wherein, in operation, a voltage onsaid energy storage capacitor has a sinusoidal voltage component attwice a frequency of said ac mains, the method comprising: controllingan amplitude of an ac current provided to said ac mains power supplyoutput such that an amount of power transferred to said ac mains powersupply output is dependent on an amplitude of said sinusoidal voltagecomponent on said energy storage capacitor, wherein said controlling isperformed by controlling a dc-to-ac converter converting energy storedin said energy storage capacitor to ac power for said ac mains powersupply input; and tracking a maximum power point of said dc source bycontrolling said dc-to-ac converter.

As previously described, broadly in embodiments of such methods changingthe fluctuating sinusoidal component of voltage on the (dc link) energystorage capacitor changes the voltage at the input from the dc powersource and the current (from the dc power source) is forced to followthe change in voltage, in accordance with the current—voltagecharacteristic of the dc power source. If power is drawn from the dclink and provided to the ac mains output the dc voltage on the energystorage capacitor drops and the dc input voltage drops concomitantly(and vice versa). Thus in embodiments of the method sensing (just) thevoltage on the energy storage capacitor can be employed to control bothcurrent and voltage at the input of the power conditioning unit.

The invention also provides a carrier such as non-volatile memorystoring processor control code for controlling a processor to implementa method as described above.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will now be further described,by way of example only, with reference to the accompanying figures inwhich:

FIG. 1 shows an example power conditioning unit suitable forimplementation of an MPPT tracking system according to an embodiment ofthe present invention.

FIG. 2 shows details of the power conditioning unit suitable of FIG. 1.

FIG. 3 shows the DC capacitor voltage in the power conditioning unit ofFIG. 1.

FIG. 4 shows control block A in the power conditioning unit of FIG. 1.

FIG. 5 shows example characteristics of a photovoltaic panel array asknown in the art.

FIG. 6 shows control block B in the power conditioning unit of FIG. 1.

FIG. 7 shows details of examples of control blocks A and B for the powerconditioning unit of FIG. 1.

FIG. 8 shows output and input powers for the power conditioning unit ofFIG. 1.

FIG. 9 shows further example characteristics of a photovoltaic panelarray.

FIG. 10 shows a block diagram of an example dc input portion of aphotovoltaic power conditioning unit incorporating an MPPT trackingsystem according to an embodiment of the invention.

FIG. 11 shows a block diagram of an example ac output portion of aphotovoltaic power conditioning unit incorporating an MPPT trackingsystem according to an embodiment of the invention.

FIG. 12 shows a circuit diagram of an example dc input portion of aphotovoltaic power conditioning unit incorporating an MPPT trackingsystem according to an embodiment of the invention.

FIG. 13 shows details of an ac output portion of a photovoltaic powerconditioning unit incorporating an MPPT tracking system according to anembodiment of the invention.

FIG. 14 shows the voltage on a DC link capacitor voltage in aphotovoltaic power conditioning unit incorporating an MPPT trackingsystem according to an embodiment of the invention, illustrating asinusoidal component of the voltage.

FIG. 15, shows an example control procedure for the power injectioncontrol block of a power conditioning unit with maximum power pointtracking according to an embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Power Conditioning Units

We first describe examples of photovoltaic power conditioning units inthe context of which embodiments the MPPT (maximum power point tracking)techniques we describe may advantageously employed.

Thus we will first describe a method to control direct current energysources, in particular a method to control direct current energy sourcesthat utilise power electronics converters to condition the input powerinto alternating current electricity that is supplied to the mains. Suchpower electronics converter comprises of a plurality of conversionstages and one energy reservoir in the form of a capacitor. The methodpresented allows the utilisation of long-lifetime polyester orpolypropylene capacitors as opposed to short-lifetime electrolyticcapacitors. The method uses two control algorithms: one algorithmcontrols the power extracted from the energy source that is supplied tothe energy reservoir and another controls the transfer of power from thereservoir into the electricity mains.

In one arrangement there is provided a power conditioning unit fordelivering power from a dc power source to an ac mains power supplyoutput, the power conditioning unit comprising: a power conditioningunit input for receiving power from said dc power source; a powerconditioning unit output for delivering ac power; an energy storagecapacitor; a dc-to-dc converter having an input connection coupled tosaid power conditioning unit input and an output connection coupled tothe energy storage capacitor; and a dc-to-ac converter having an inputconnection coupled to said energy storage capacitor and an outputconnection coupled to said power conditioning unit output; wherein saidenergy storage capacitor is a non-electrolytic capacitor; and whereinsaid power conditioning unit comprises two control blocks, a first,power extraction control block to control said dc-to-dc converter tocontrol power extracted from said dc power source and provided to saidenergy storage capacitor, and a second, power injection control block tocontrol said dc-to-ac converter to control power injected into said acmains power supply from said energy storage capacitor; and wherein saidpower extraction control block has an input coupled to said powerconditioning unit input for receiving power from said dc power sourceand has an output to control said dc-to-dc converter and is configuredto regulate a voltage of said dc power source to control power extractedfrom said dc power source to said energy storage capacitor.

The ac mains power supply output may be connected to the utility grid,so that the power conditioning unit delivers power into the grid, or itmay be a standalone power supply output for supplying power toelectrical appliances.

The dc-to-ac converter may be configured to deliver a substantiallysinusoidal current or voltage to the ac mains power supply outputregardless of a voltage on the energy storage capacitor. This may beachieved by maintaining a current or voltage to the power supply outputsubstantially at a reference sinusoid current or voltage. This maycomprise controlling transistors in the dc-to-ac converter responsiveboth to a voltage or current from the energy storage capacitor and tothe current or voltage to the power supply output.

The energy storage capacitor may comprise a non-electrolytic capacitorsuch as a film-type capacitor (for example polyester or polypropylene).The value of the capacitance may be directly proportional to the maximumpower transfer capability, that is, the rated power of the apparatus.This value may be lower than that of the capacitor in a conventionalpower conditioning unit with the same power rating. For example, lessthan 20 microfarads, less than 15 microfarads, less than 10 microfarads,less than 5 microfarads or another size available for a non-electrolyticcapacitor.

We will also describe a dc-to-dc converter for delivering power from adc power source to a dc output, the converter being configured tomaintain a voltage on the dc power source substantially constant over arange of dc output voltages, the converter comprising an input forreceiving power from said dc power source, an output for delivering dcpower, at least one power device for transferring power from the inputto the output, a sensing circuit for sensing a voltage on said input,and a driver circuit for driving said at least one power deviceresponsive to said sensing to control said power transfer.

We will also describe an inverter for delivering power from a dc powersource to an ac output, the inverter being configured to maintain asubstantially sinusoidal output voltage or current over a range of dcpower source voltages, the inverter comprising an input for receivingpower from said dc power source, an output for delivering ac power, atleast one power device for transferring power from the input to theoutput, a low-pass filter coupled to said input, a sensing circuit forsensing an output from the low-pass filter and comparing with areference, and a driver circuit for driving said at least one powerdevice responsive to said sensing to control said power transfer.

We will also describe a power conditioning unit for delivering powerfrom a dc power source to an ac mains power supply output, wherein alink capacitor of the power conditioning unit connected in parallelbetween an output of a dc-to-dc converter of said power conditioningunit and an input of a dc-to-ac converter of said power conditioningunit is not an electrolytic capacitor.

We will also describe a method of controlling a power conditioning unitfor delivering power from a dc source into an ac electricity supply, thepower conditioning comprising: an input for connecting the dc powersource; an output for connecting the electricity supply; a first,dc-to-dc power conversion stage for voltage conditioning of the dc powersource; a second power conversion stage for power injection into the acelectricity supply; and a dc link energy storage capacitor for energybuffering from the dc power source to the electricity supply; whereinthe method comprises controlling said second power conversion stage tocontrol an amplitude of an ac current provided to said ac electricitysupply output such that an amount of power transferred to said ac mainspower supply output is dependent on a peak amplitude of a fluctuatingsinusoidal component of a dc voltage on said energy storage capacitor.

Thus an example power conditioning unit uses a system for controllingthe transfer of power from a dc energy source, such as a solar panel,fuel cell, dc wind turbine, etc, into the electricity mains supply, andin particular allows the replacement of short-lifetime energy reservoirsby long-lifetime polyester or polypropylene capacitors.

The energy control and MPPT techniques we describe can be used in anypower electronics converter device (1) as shown in FIG. 1. Thisapparatus (1) is made of three major elements: a power converter stage A(3), one reservoir capacitor C_(dc) (4), and one power converter stage B(5). The apparatus (1) has a plurality of inputs connected to a directcurrent (dc) source, such as a solar or photovoltaic panel array (2)comprising one or more dc sources connected in series and/or inparallel. The apparatus (1) is also connected to the electricity supply(6) so that the energy extracted from the dc source (1) is transferredinto the mains (6).

The power converter stage A (3) may be of different types: it can be astep-down converter where the voltage at the input is decreased usingsome power electronics topology; it can be a step-up converter where theinput voltage is amplified using a different type of power electronicscircuit; or it can do both amplify and attenuate the input voltage. Inaddition, it may provide electrical isolation by means of a transformeror a coupled inductor. In whatever case, the electrical conditioning ofthe input voltage should be such that the voltage across the capacitorC_(dc) (4) remains higher than the grid voltage (6) magnitude at alltimes. Also, this block contains one or more transistors, inductors, andcapacitors. The transistor(s) are driven through a pulse widthmodulation (PWM) generator. The PWM signal(s) have variable duty cycle,that is, the ON time is variable with respect to the period of thesignal. This variation of the duty cycle effectively controls the amountof power transferred across the power converter stage A (3).

The power converter stage B (5) injects current into the electricitysupply (6). Therefore, the topology utilises some means to control thecurrent flowing from the capacitor C_(dc) (4) into the mains (6). Thecircuit topology can be either a voltage source inverter or a currentsource inverter.

FIG. 2 shows an example of a power conditioning unit to which thecontrol system of FIG. 1 may be applied. In FIG. 2 Q1-Q4, D1-D4 and thetransformer form a voltage amplifier; Q9, D5, D6 and Lout performcurrent shaping; and Q5-Q6 constitute an “unfolding” stage. Control A (7in FIG. 1) may be connected to the control connections (e.g. gates orbases) of transistors in power converter stage A (21) to control thetransfer of power from the dc energy source (20). The input of thisstage is connected to the dc energy source and the output of this stageis connected to dc link capacitor 22. This capacitor stores energy fromthe dc energy source for delivery to the mains supply (24). Control Amay be configured to draw a substantially constant power from the dcenergy source regardless of the dc link voltage V_(dc) on C_(dc).

Control B (8 in FIG. 1) may be connected to the control connections oftransistors in power converter stage B (23) to control the transfer ofpower to the mains supply. The input of this stage is connected to thedc link capacitor and the output of this stage is connected to the mainssupply. Control B may be configured to inject a substantially sinusoidalcurrent into the mains supply regardless of the dc link voltage V_(dc)on C_(dc).

The capacitor C_(dc) (4) acts as an energy buffer from the input to theoutput. Energy is supplied into the capacitor via the power stage A (3)at the same time that energy is extracted from the capacitor via thepower stage B (5). The system provides a control method that balancesthe average energy transfer and allows a voltage fluctuation, resultingfrom the injection of ac power into the mains (6), superimposed to theaverage dc voltage of the capacitor C_(dc) (4), as shown in FIG. 3. Thefigure shows an average voltage of 475V and a 100 Hz fluctuation of peakamplitude of 30V. The peak amplitude depends on the amount of powerbeing transferred from the input (2 in FIG. 1) to the output (6). Thefrequency of the oscillation can be either 100 Hz or 120 Hz depending onthe line voltage frequency (50 Hz or 60 Hz respectively).

Two synchronised and independent control blocks control the system (1):a control block A (7) that directly controls the power stage A (3), anda control block B (8) that directly controls the power stage B (5).

Control block A (7) has the configuration shown in FIG. 4. It comprisesan adder (31), a negative proportional gain (32), a PWM generator (33),the system plant (34), and a feedback gain (35). This control blockregulates the voltage across the dc source (2). This voltage, v_(in), ismeasured and adjusted by gain k₁ (35). It is then subtracted to avoltage reference, V_(ref), using the adder (31). The error, (v_(ref)−k₁v_(in)), is then amplified by a factor of −k₂. The resulting signal isnegatively proportional to the error. Therefore, a positive errorgenerates a decrement in the driving signal and conversely. This drivingsignal is input to a PWM generator (33) that can be a microcontroller,or a PWM integrated circuit. This block generates digital pulses that,in turn, drive the transistors of the power stage A (3) that isequivalent to the plant (34).

Controlling the dc source (2) voltage directly controls the power beingtransferred across power stage A (3) as is shown in FIG. 5 for aphotovoltaic panel array.

Control block B (8) has the configuration shown in FIG. 6. It comprisesan adder (41), a sample and hold (SH) with period T block (42), aproportional-derivative (PD) compensator (43), the system plant (44), alow-pass filter (LPF) feedback block (45). This control block regulatesthe average voltage across capacitor C_(dc) (4). Because the voltage,v_(dc), contains the sum of a constant voltage and a fluctuatingsinusoidal component, the signal is scaled and filtered using the LPFblock (45). This generates a constant voltage that is compared against areference, V_(dc) _(_) _(ref), using adder (41). The error is measuredevery T seconds using a Sample and Hold, SH, block (42). The resultingsampled error is forwarded to a PD compensator (43) that sets theamplitude of the current injected to the mains (6) via power stage B(5). The update of this current reference, I_(ref), amplitude is doneevery T seconds, which is the inverse of the line voltage frequency.Hence, it can take the values of 0.02 or 0.0167 seconds for a linefrequency of 50 or 60 Hz respectively. This is needed in order toprevent current injection distortion.

An implementation of control blocks A and B is shown in FIG. 7. Bothblocks operate independently but share a common microcontroller forsimplicity. The microcontroller performs the control strategy depictedin FIG. 6 for block B. In addition the microcontroller could incorporatesome means of maximum power point tracking control in case the inputsource is a photovoltaic panel in block A in order to generate areference input voltage used in FIG. 4. Consequently the input voltageand current and the dc-link voltage are fed into the microcontroller viaan arrangement of operational amplifiers or signal conditioning blocks.

The control shown in FIG. 4 for block A is implemented using analogueelectronics in the form of operational amplifiers and the phase-shiftPWM controller depicted in FIG. 7 (51). As mentioned before, the inputvoltage reference is obtained through the microcontroller via a digitalto analogue converter (DAC). The proportional error is obtained insidethe phase-shift PWM controller that, in turn, generates PWM signals forthe transistors of stage A (21).

Implementation of control B (52) includes a current transducer thatsenses the rectified output current. This signal is conditioned toappropriate voltage levels using operational amplifiers and is thencompared against a reference current. The reference current is generatedin the microcontroller by an algorithm shown in FIG. 6 and the resultingdigital word is sent to a DAC in order to get an analogue,instantaneous, current reference. Changes to the current magnitude aredone in a periodic basis (with period equal to the grid voltage period)in order to avoid current distortion. The result of the comparisonbetween the reference and the actual current is buffered through a Dflip-flop which, in turn, drives transistor Q9 in FIG. 2. TransistorsQ5-Q8 form a full-bridge that switches at line frequency using ananalogue circuit synchronised with the grid voltage. Transistors Q5 andQ8 are on during the positive half cycle of the grid voltage and Q6 andQ7 are on during the negative half cycle of the grid voltage.

FIG. 8 shows the output and input powers using the aforementionedcontrol. Clearly, the instantaneous power output is a sinusoidsuperimposed to an average positive value. In contrast, the input isconstant throughout the period of the line voltage. The power differencecreates an energy mismatch that is absorbed in capacitor C_(dc). Thiseffectively appears as a fluctuation across the capacitor, as is shownin FIG. 3.

MPPT (Maximum Power Point Tracking) Techniques

We will describe a method and system for tracking the maximum powerpoint of an energy generator and extracting maximum power from such agenerator when coupled to the load. In embodiments the method/systemcomprises two independent control blocks. The first block controls thevoltage amplification stage that interfaces with the energy generator.The energy generator is preferably a solar module. In embodiments thefirst control block does not function to regulate the amount of energyto be transmitted but functions only as a switch, either allowing energyflow or preventing any energy flow from the generator and through theamplification stage, regardless of the amount. The output of the voltageamplification stage is coupled to an energy reservoir capacitor. Energyflow is therefore dependent on the amount of “room” (the amount ofadditional energy which can be stored) in the reservoir capacitor. Thesecond control block is a feedback control loop that interfaces theenergy reservoir capacitor to the coupled output load. The secondcontrol block regulates the amount of power to be injected into the loadby emptying the energy reservoir capacitor. The second control blockuses, in embodiments exclusively, the level of voltage fluctuations onthe energy reservoir (storage capacitor) to control the amount of powerbeing extracted from the energy generator and also the amount of powerbeing injected into the load. In embodiments no use of (measured)current values is made. Thus in embodiments the maximum power pointtracking uses two completely independent loops and uses exclusivelyvariations characteristic of the reservoir capacitor.

Some energy generators, such as solar photovoltaic cells, constitute anon-linear power characteristics profile such as one illustrated in FIG.9. In the figure maximum power is harvestable at the point labelled X,which exhibits maximum power point current Imp and voltage Vmp. It ispreferable that the operating point that yields most energy is attainedand maintained. The method we describe does not use the voltage andcurrent values measured at the output of the generator to performmaximum power point tracking. Instead the method measures the voltagefluctuations in the DC link and uses the measured values to track themaximum power point.

Referring to FIGS. 10 and 11, these show a block diagram of input 1002and output 1004 stages of an embodiment of a solar PV power conditioningsystem 1000 incorporating an MPPT control methodology for the dc inputside of the power conditioning unit according to an embodiment of theinvention. Thus FIG. 10 shows an energy generator 1010 such as one ormore PV panels feeding a voltage amplification stage 1012 with asubstantially constant amplification factor (which may be less than,equal to, or greater than unity depending, for example, on whether thedc input is from a single PV panel or a string of series connectedpanels). This in turn provides power to an energy reservoir 1014, inembodiments a storage capacitor coupled to a dc link between the input,voltage amplification stage and an output, voltage inversion stage.Control block A 1016 controls voltage amplification stage 1012, but inembodiments only to switch power from the energy generator on and offinto the energy reservoir. In embodiments control block A does notprovide a variable gain control and simply comprises a fixed frequencyoscillator. Voltage inversion stage 1018 has an input coupled to theenergy reservoir 1014 and provides an ac mains output to load 1020, forexample via a grid connection. Control Block B 1022 monitors the voltageon the dc link via sense connection 1022 a (but in embodiments does notsense the current on this link), and the current into and voltage on theload via sense connections 1022 b,c (in embodiments connection 1022 c iswithin the power conditioning unit), and provides gate drive outputsignals 1022 d for controlling the voltage inversion (“unfolding”) stage1018, more particularly for controlling the power drawn from the energyreservoir and provided into the load via the grid. The gate drivesignals 1022 d are sequenced to control the power converter switches ofthe power conversion stage 1018 (see also FIG. 2); this provides aconvenient technique for controlling the switching frequency of thisstage.

In FIG. 10, control block A functions as a power switch, allowing powerto flow from the energy generator to the voltage amplification stage (oreffectively switching the voltage amplification stage on/off or in/out).Control block A can also be set to turn off power from the energygenerator in the event of over-voltage and under-voltage conditions.

The voltage amplification stage can have a fixed amplification ratio ora selectable or multiplexable ratio such as may be provided by a tappedtransformer. The voltage amplification stage may comprise a half-bridge,a full bridge, a push-pull or a similar voltage inversion stage. Such aninversion stage may comprise semiconductor switching devices such asMOSFETs. The voltage inversion stage can be coupled to a transformer,whose amplification ratio results in a desired voltage in the DC linkreservoir capacitor, for example of order 400 volts. The output of thetransformer is coupled to a rectifier stage. An inductor may be includedbetween the rectifier bridge and the DC link reservoir capacitor.

Depending on the input voltage the voltage amplification stage 1012 mayprovide an amplification in the range ×5 to ×20, for example around ×12for a dc input voltage of ˜35 volts, giving a dc link voltage of around420 volts.

FIG. 12 shows a more detailed circuit diagram of an example input stage1002 implementing the control methodology we describe. The energygenerator may be a solar module or a group of solar modules. In thisexample the voltage amplification stage comprises a half-bridge, whichin turn is made up of two series switches (MOSFETs), Q1 and Q2, and twoseries capacitors C1 and C2, and the transformer TX1. A rectifier bridge1013 made up of diodes is coupled to the output of the transformer. Therectifier bridge is itself coupled to the DC link capacitor Cd via afilter inductor Ld. The control block in FIG. 12 produces a constantduty cycle PWM signal, and hence no modulation is implemented. In theevent that Cd is full, defined as the voltage across it being equal orlarger than the rectified output from transformer secondary, no powerflows into Cd even though Q1 and Q2 are switched on and offcontinuously. Hence control block A does not regulate the amount ofpower extracted from the generator.

FIG. 13 shows a more detailed circuit diagram of an example output stage1004 implementing the control methodology we describe. Referring to FIG.13, control block B measures the voltage fluctuations in the DC linkthat are used for regulation of the amount of power being harvested fromthe energy generator and therefore the amount of power injected into theload. A preferred load is the utility grid. In the case of the gridload, control B measures the peak and trough voltages on the DC linkcapacitor via a scaling circuit (the potential divider circuit of R3 andR4). The scaled values of the peak Vp and the trough Vt voltages areused to compute the amount of power flowing through the capacitor (asdescribed below). In embodiments the voltage sense connection to ControlBlock B is via a rectifier).

Energy Storage and DC Link Capacitance

Due to the AC nature of the power being transferred into the grid andthe current-voltage characteristic of the power being generated by thesolar module, energy storage is essential in a PV inverter if maximumpower is to be harvested from the solar module. In our preferred design,energy storage is delegated to the DC link reservoir capacitor. Theamount of power transferred into the grid is related to the energychange in the capacitor and therefore the voltage ripple on thecapacitor. One advantage of implementing energy storage on the DC linkis that a large ripple can be allowed on the capacitor. Equation 1illustrates the relationship between energy change, the capacitance andthe voltage on the capacitor:U _(R)=½C _(dc)(V _(P) ² −V _(T) ²)  (1)where V_(P) is the capacitor peak voltage and V_(T) is the capacitortrough voltage. The power transferred would be the energy change persecond. FIG. 14 illustrates the fluctuation (sinusoidal ripple) on theDC link capacitor.

Thus block B automatically achieves MPPT by regulating the amount ofinjected current with reference to (dependent on) the dc link voltagefluctuation.

However, the MPPT tracking technology we have described is notrestricted to operating in the context of a power conditioning unitwhich deliberately allows (and controls based on) a degree of ac rippleon the dc link. It may therefore be helpful to enlarge upon thedescription of the operation of embodiments of the technique.

Consider an input current and voltage I, V to the inverter provided by aphotovoltaic power source, a dc link current and voltage I_(d), V_(d),and a output current and voltage into grid mains of I_(grid), V_(grid).Since V_(grid) is approximately constant, the power injected into thegrid mains is proportional to I_(grid). Also, absent losses, the inputpower I.V=I_(d)·V_(d). and thus I_(d)·V_(d) determines the point on thephotovoltaic IV characteristic at which the system operates. Broadlyspeaking the system senses the ripple on V_(d) which, in embodiments,(as described above) is a measure of the power flowing through the dclink. More particularly the system controls the output “unfolding” stage(for example a buck stage converter) to maximise the level (amplitude)of this ripple component on the dc link voltage/energy storagecapacitor, and hence also to maximise the power injected into the acmains. (The skilled person will appreciate that V_(d) on its own doesnot provide a good measure of the power on the dc link).

In a preferred implementation the control block 1022 generates a halfsinusoidal template voltage (with an amplitude varying between zero and3.3 volts) in phase with the grid, for comparison with a (rectified)version of the sensed load current 1022 b. The sensed load voltage 1022c is used only to determine the phase of the ac mains. The amplitude ofthe template is adjusted dependent on the level of ripple sensed on theenergy storage capacitor/dc link (via line 1022 a). If the templateamplitude is greater than the amplitude of the sensed grid current theswitching frequency is increased to inject more power into the grid, andvice versa. Thus, broadly speaking, the amplitude of the template isadjusted dependent on the dc link ripple and the output current iscontrolled to match the template amplitude.

Referring now to FIG. 15, this shows an example control procedure forcontrol block B 1022. FIG. 15 is an example; the skilled person willappreciate that many variations are possible.

Presuming that the procedure begins at start-up of the inverter, theprocedure first initialises the amplitude of the template signal to anarbitrary, relatively low value, for example 0.5 volts on the previous0.-3.3 volts scale (step S1500). Referring again to FIG. 9, at thispoint the output voltage from the photovoltaic panel is at a maximum andthe output current is at substantially zero; the level of ripple on thedc link is also substantially zero.

The procedure determines the phase of the ac grid mains voltage (S1502)and synchronises the half-sinusoidal template to the grid. The procedurethen senses the grid current (S1504), for example by sensing the voltageacross a current sense resistor; at start-up this will be approximatelyzero. The procedure then determines an error value E from the differencebetween the template and the sensed grid current (S1506), which atstart-up (continuing the previous example) will be 0.5. The procedurethen determines a switching rate for the voltage inversion stage 1018dependent upon this error, in one example algorithm increasing theswitching rate if E is positive and decreasing the rate if E isnegative. Thus in the present example, at start-up the templateamplitude is greater than that of the sensed grid current so theswitching rate is increased. This increases the current (and hencepower) injected into the ac grid mains, so that the ripple voltage onthe dc link also increases.

At step S1510 the procedure measures the ripple voltage on the dc linkand, at step S1512, adjusts the template amplitude dependent on thismeasurement, more particularly increasing the amplitude if the ripplevoltage increased, and vice versa. The procedure then loops back to stepS1504 to once again sense the current being injected into the ac mains.Thus, for example, if the error is positive the template amplitudeincreases so that it is once again greater than the amplitude of thesensed current injected into the grid, and thus the switching rate ofthe voltage inversion stage is once again increased. However if theprevious change decreased the measured ripple voltage (which senses thepower drawn from the photovoltaic panel), then the template amplitude,and hence switching rate of the voltage inversion stage, is alsodecreased. In this way the control technique operates to control theoutput voltage inversion stage such that the photovoltaic panel ismaintained at substantially its maximum output power point.

We have thus described a power conditioning unit with MPPT for aphotovoltaic panel in which a power injection control block has a senseinput coupled to an energy storage capacitor on a dc link and controls adc-to-ac converter to control the injected mains power. The powerinjection control block tracks the maximum power point by measuring asignal on the dc link which depends on the power drawn from the dc powersource, and thus there is no need to measure the dc voltage and currentfrom the PV panel. In embodiments the signal is a ripple voltage leveland the power injection control block controls an amplitude of an accurrent output such that an amount of power transferred to the gridmains is dependent on an amplitude of a sinusoidal voltage component onthe energy storage capacitor.

Preferred embodiments of the MPPT tracking system work with an inverterof the general type described above, but the techniques may also be usedwith other types of inverter, for example a ‘four-switch’ inverter asdescribed in our U.S. Pat. No. 7,626,834, in particular if this isprovided with a half or full bridge dc boost stage (with a transformer)at the front end.

Preferred embodiments of the invention are employed in inverters forphotovoltaic panels, but in principle embodiments of the invention mayalternatively be employed for MPPT for fuel cells. No doubt many othereffective alternatives will occur to the skilled person. It will beunderstood that the invention is not limited to the describedembodiments and encompasses modifications apparent to those skilled inthe art lying within the scope of the claims appended hereto.

We claim:
 1. A method of maximum power point tracking (MPPT) in a powerconditioning unit for delivering power from a DC power source to an ACcircuit output, the power conditioning unit including an energy storagecapacitor for storing energy from the DC power source and a DC to ACconverter delivering AC power to the AC circuit output, the methodcomprising: tracking a maximum power point (MPP) of the DC power sourceby controlling the DC to AC converter to convert energy stored in theenergy storage capacitor to AC power for the AC circuit output, whereintracking an MPP comprises: sensing, at a circuit node coupled to theenergy storage capacitor, a signal responsive to a level of power drawnfrom the DC power source; determining whether the level of power isincreasing or decreasing relative to a previously sensed level of power;and controlling a frequency of the DC to AC converter to maintainoperation of the power conditioning unit at the MPP based at least inpart on whether the level of power is increasing or decreasing.
 2. Themethod of claim 1, wherein the signal is an amplitude of a ripplevoltage on the energy storage capacitor.
 3. The method of claim 2,wherein the ripple voltage has a sinusoidal voltage component at twice afrequency of the AC circuit output.
 4. The method of claim 3, furthercomprising determining a power transferred from the DC power source tothe AC output using a squared value of the sinusoidal voltage component.5. The method of claim 3, wherein the power conditioning unit controlsthe DC to AC converter to substantially maximize the amplitude of theripple voltage on the energy storage capacitor.
 6. The method of claim1, wherein the AC circuit output is coupled to an AC grid having asinusoidal voltage.
 7. The method of claim 6, wherein the powerconditioning unit controls the DC to AC converter to match the ACcircuit output to the sinusoidal voltage of the AC grid by generating avoltage template that is in phase with the AC grid.
 8. The method ofclaim 7, wherein the power conditioning unit controls the AC to DCconverter responsive to a difference between the voltage template and anamplitude of a ripple voltage on the energy storage capacitor.
 9. Themethod of claim 1, wherein the power conditioning unit further comprisesa DC voltage amplifier coupled between the DC power source and theenergy storage capacitor to increase a voltage from the DC power sourceby a substantially constant amplification factor.
 10. A method ofmaximum power point tracking (MPPT) in a power conditioning unit fordelivering power from a DC power source to an AC mains, the powerconditioning unit including an energy storage capacitor for storingenergy from the DC power source and for delivering energy to the ACmains, the method comprising: tracking a maximum power point (MPP) ofthe DC power source by controlling a DC to AC converter convertingenergy stored in the energy storage capacitor to AC power for the ACmains, wherein tracking the MPP comprises: sensing, at a circuit nodecoupled to the energy storage capacitor, a signal responsive to a levelof power drawn from the DC power source; and controlling the DC to ACconverter to adjust an amplitude of an AC output of the powerconditioning unit to substantially maximize the sensed signal.
 11. Amethod as claimed in claim 10, wherein controlling the DC to ACconverter comprises controlling a switching rate of the DC to ACconverter.
 12. The method of claim 10, wherein the signal is anamplitude of a ripple voltage on the energy storage capacitor.
 13. Themethod of claim 12, wherein the ripple voltage has a sinusoidal voltagecomponent that is twice a frequency of the AC mains.
 14. The method ofclaim 13, further comprising determining a power transferred from the DCpower source to the AC mains using a squared value of the sinusoidalvoltage component.
 15. The method of claim 10, wherein the powerconditioning unit further comprises a DC voltage amplifier coupledbetween the DC power source and the energy storage capacitor to increasea voltage from the DC power source by a substantially constantamplification factor.
 16. A method of maximum power point tracking in apower conditioning unit for delivering power from a DC power source toan AC mains, the power conditioning unit including an energy storagecapacitor for storing energy from the DC power source and for deliveringenergy to the AC mains, wherein in operation a voltage on the energystorage capacitor has a sinusoidal voltage component at twice afrequency of the AC mains, the method comprising: controlling anamplitude of an AC current provided to the AC mains such that an amountof power transferred to the AC mains is dependent on an amplitude of thesinusoidal voltage component on the energy storage capacitor, whereinthe controlling is performed by controlling a DC to AC converterconverting energy stored in the energy storage capacitor to AC power forthe AC mains; and tracking a maximum power point of the DC power sourceby controlling the DC to AC converter.
 17. A method as claimed in claim16, wherein the tracking is performed without measuring a DC voltage orDC current provided from the DC power source.
 18. The method of claim16, wherein the DC to AC converter is controlled to substantiallymaximize an amplitude of the sinusoidal voltage component on the energystorage capacitor.
 19. A method as claimed in claim 16, furthercomprising determining a power transferred from the DC power source tothe AC mains using a squared value of the sinusoidal voltage componenton the energy storage capacitor, and wherein the tracking is responsiveto the determined power.
 20. A method as claimed in claim 16, whereinthe tracking is performed by a power injection control block controllingpower injected into the AC mains power supply by controlling a switchingfrequency of the DC to AC converter.